John Summerfield arrives at court. The headteacher was fined £20,000 for 'an act of folly' that led to a pupil suffering serious injuries. Photograph: Dave Thompson/PA

A headteacher was fined £20,000 today for an "act of folly" during A-level celebrations which left one of his pupils with a fractured skull and permanent damage to his eyesight.

John Summerfield was told by a judge that he had forfeited his good name in education by taking "slightly inebriated" teenagers on to the roof of Sacred Heart Catholic college in Crosby, Merseyside, where one of them fell through a skylight.

The 65-year-old, who was also ordered to pay £22,000 legal costs, was convicted by a jury last month of breaching the Health and Safety at Work Act by flouting school rules which put the roof out of bounds. The sentence hearing at Liverpool crown court accepted that his motives were well-intentioned but ignored the risks involved.

The court was told that during a celebration party thrown by the school, Summerfield took a dozen sixth formers through two locked doors on to the roof to get a bird's-eye view of renovation work. He warned them to keep away from skylights but Joel Murray, then 18, trod on one and fell through, landing in a corridor 2.4 metres (8ft) below.

The teenager fractured his skull, broke several ribs, perforated an eardrum and damaged both eyes. Judge Nigel Gilmour QC was told the injuries would limit Murray's employment options throughout his working life.

Kevin Donnelly, prosecuting, told the hearing that the incident in August 2008 came at the height of the party, following the release of A-level results earlier in the day.

"Nobody was drunk but it is possible they were slightly affected by drink, possibly slightly inebriated," he said.

"Mr Summerfield took them into an area which was normally out of bounds. The decision was his and his alone. The very act of taking the students to that area was a breach of his duty of care. He failed to give appropriate consideration to the risks involved."

Patrick Cassidy, in mitigation, said Summerfield had enjoyed an "impeccable" teaching career for 42 years. He had lost his good character among colleagues because of the very serious, but single, misjudgment.

Judge Gilmour told Summerfield: "This was an act of folly. You didn't know the precise risk of the rooflight but you did know it was dangerous or at least potentially dangerous.

"This was a grave and uncharacteristic judgment which led to a pupil suffering serious injury, the consequences have been quite serious too. But this act of isolated carelessness is not a true reflection of your distinguished career. I hope that you will be able to forget it."

Summerfield, who was given six months to pay the fine and costs, nodded at the judge as the sentencing finished. If he fails to pay, he could face a year in prison.

Mike Sebastian, principal inspector for the Health and Safety Executive on Merseyside, said: "The roof was kept out of bounds for a reason. As the headteacher, Mr Summerfield should have thought about the possible consequences before deciding to take them through two locked doors on to the roof.

"Students should expect to be in a safe environment when they're at school and look to their teachers for guidance on what is and what isn't safe. Sadly, a pupil suffered serious injuries because of the poor judgment of his headteacher, and is unlikely to ever fully recover."